Oops, I don't know how I managed to skip answering this.
Even if it's more than a month late, someone might find it useful sometime...
Anyway, you connect whatever device you want to record from to the HD PVR's component-video input, and connect the HD PVR's USB output to a USB port on the Mac. Launch the EyeTV software. When you turn on the HD PVR, EyeTV recognizes it as an input device just like it does Elgato's own EyeTV USB tuners, except that EyeTV's controller displays "Component Input" instead of a channel number. Hit the "record" button on the controller to start recording.
On a Mac, you can also use the HDPVRCapture application that I mentioned. For PCs, Hauppauge includes capture software with the HD PVR.
I use this setup only for dubbing recordings off a Sony HD DVR for archiving, so I haven't tried to schedule unattended recordings off a HD tuner or cable box, i.e. use it as a substitute for a DVR. For that, the computer has to be able to change channels on the tuner or cable box via an IR blaster.